A/B Testing

Convert experiments

Conversion rate optimization (CRO) experiments on bedrock are run on separate, experimental versions of our main landing pages using a tool called Convert. Convert experiments are for relatively simple multivariate experiments, such as testing changes to headlines, images, or button copy.

Experimental versions of our main product pages can be found under the /exp/ app within bedrock. We keep our main product pages separate from experiment code for the following reasons:

  • Performance (when an experiment is active, only those who qualify need download the Convert JS).

  • Security (keeping the experimentation surface as small as required).

Creating experimental pages

Templates within the /exp/ app should have the following characteristics:

  1. They should extend their respective jinja templates and live at a separate URL (e.g. /exp/firefox/new/ should extend /firefox/new/).

{% extends "firefox/new/desktop/download.html" %}
  1. They should override the following blocks required to load Convert

{% block html_attrs %}{{ super() }} {% include 'exp/includes/id.html' %}{% endblock %}

{% block experiments %}
    {% include 'exp/includes/convert.html' %}
{% endblock %}

{% block stub_attribution %}
    {% include 'exp/includes/stub.html' %}
{% endblock %}
  1. If a page is indexed by search engines, then the experimental equivalent should have a canonical URL that points back to the original page. If the page is considered to be an in-product page, then the experimental page should retain the same noindex meta tag.

{% block canonical_urls %}
    <link rel="canonical" href="{{ settings.CANONICAL_URL }}/{{ LANG }}/firefox/new/">
{% endblock %}
  1. Experimental pages should also contain an Open Graph sharing URL that points back to the original page.

{% block page_og_url %}{{ settings.CANONICAL_URL }}/{{ LANG }}/firefox/new/{% endblock %}

Once an experimental page exists, A/B tests can then be implemented using the Convert dashboard and editor. Convert experiments should be coded and tested against staging, before being reviewed and scheduled to run in production.

QA for Convert experiments

The process for QA’ing Convert experiments is as follows:

  1. Experiments are completely built and configured to run on https://www.allizom.org/*

  2. In the Github issue for an experiment, someone will request review by an engineer.

An engineer reviewing the experiment will:

  1. Verify that the experiment is not configured to run on https://www.mozilla.org/ (production) yet.

  2. Activate the experiment to run on stage.

During review, the engineer will compare the following to the experiment plan:

  1. The experiment’s logic.

  2. Any JS included (in Convert editor’s JS field).

  3. Any CSS included (in Convert editor’s CSS field).

  4. The target audience is configured.

  5. The goals are configured.

  6. The distribution percentages are configured.

  7. The target URLs are configured.

Once the engineer is satisfied, the engineer (or someone else with write privileges) will:

  1. Add https://www.mozilla.org/* to the list of urls the experiment can run on.

  2. Reset the experiment (eliminating any data gathered during QA).

  3. Activate (or schedule) the experiment.


* should be replaced by the exact URL pathname for the experiment page.

Routing traffic to experimental pages

Once an experimental page has been created, a predefined percentage of traffic can then be redirected from the main product page, to the experimental page using a Cloudflare Worker script. We do this using a worker because it has significant performance improvements over doing redirection on the client or the server, and can also be done independently of a bedrock deployment.

The code for redirecting to experimental pages lives in the www-workers repository.

Adding a new redirect requires making a pull request to add a new object to the experimentalPages array in the workers/redirector.js file. An existing configuration to route 6% of traffic from /firefox/new/ to /exp/firefox/new/ can be seen below:

const experimentPages = [
        'targetPath': `/en-US/firefox/new/`,
        'sandboxPath': `/en-US/exp/firefox/new/`,
        'sampleRate': 0.06


When implementing new changes to the redirector, make sure to test and verify that things are working as expected on dev and stage before pushing to production. See the documentation in the www-workers repository for more information.

Traffic Cop experiments

More complex experiments, such as those that feature full page redesigns, or multi-page user flows, should be implemented using Traffic Cop. Traffic Cop small javascript library which will direct site traffic to different variants in a/b experiments and make sure a visitor always sees the same variation.

It’s possible to test more than 2 variants.

Traffic Cop sends users to experiments and then we use Google Analytics (GA) to analyze which variation is more successful. (If the user has DNT enabled they do not participate in experiments.)

All a/b tests should have a mana page detailing the experiment and recording the results.

Coding the variants

Traffic cop supports two methods of a/b testing. Executing different on page javascript or redirecting to the same URL with a query string appended. We mostly use the redirect method in bedrock. This makes testing easier.

Create a variation view for the a/b test.

The view can handle the url redirect in one of two ways:

  1. the same page, with some different content based on the variation variable

  2. a totally different page

Content variation

Useful for small focused tests.

This is explained on the variation view page.

New page

Useful for large page changes where content and assets are dramatically different.

Create the variant page like you would a new page. Make sure it is noindex and does not have a canonical url.

{% block canonical_urls %}<meta name="robots" content="noindex,follow">{% endblock %}

Configure as explained on the variation view page.

Traffic Cop

Create a .js file where you initialize Traffic Cop and include that in the experiments block in the template that will be doing the redirection. Wrap the extra js include in a switch.

{% block experiments %}
  {% if switch('experiment-berlin-video', ['de']) %}
    {{ js_bundle('firefox_new_berlin_experiment') }}
  {% endif %}
{% endblock %}


See the traffic cop section of the switch docs for instructions.

Recording the data


If you are measuring installs as part of your experiment be sure to configure custom stub attribution as well.

Including the data-ex-variant and data-ex-name in the analytics reporting will add the test to an auto generated report in GA. The variable values may be provided by the analytics team.

if(href.indexOf('v=a') !== -1) {
        'data-ex-variant': 'de-page',
        'data-ex-name': 'Berlin-Campaign-Landing-Page'
} else if (href.indexOf('v=b') !== -1) {
        'data-ex-variant': 'campaign-page',
        'data-ex-name': 'Berlin-Campaign-Landing-Page'

Make sure any buttons and interaction which are being compared as part of the test and will report into GA.

Viewing the data

The data-ex-name and data-ex-variant are encoded in Google Analytics as custom dimensions 69 and 70.

Create a custom report.

Set the “Metrics Group” to include Sessions. Configure additional metrics depending on what the experiment was measuring (downloads, events, etc.)

Set the “Dimension Drilldowns to have cd69 in the top position and cd70 in the drilldown position.

View the custom report and drilldown into the experiment with the matching name.


Write some tests for your a/b test. This could be simple or complex depending on the experiment.

Some things to consider checking:

  • Requests for the default (non variant) page call the correct template.

  • Requests for a variant page call the correct template.

  • Locales excluded from the test call the correct (default) template.

A/B Test PRs that might have useful code to reuse

Avoiding experiment collisions

To ensure that Traffic Cop doesn’t overwrite data from any other externally controlled experiments (for example Ad campaign tests, or in-product Firefox experiments), you can use the experiment-utils helper to decide whether or not Traffic Cop should initiate.

var isApprovedToRun = require('../../base/experiment-utils.es6.js').isApprovedToRun; // relative path

if (isApprovedToRun()) {
    var cop = new Mozilla.TrafficCop({
        id: 'experiment-name',
        variations: {
            'entrypoint_experiment=experiment-name&entrypoint_variation=a': 10,
            'entrypoint_experiment=experiment-name&entrypoint_variation=b': 10


The isApprovedToRun() function will check the page URL’s query parameters against a list of well-known experimental params, and return false if any of those params are found. It will also check for some other cases where we do not want to run experiments, such as if the page is being opened in an automated testing environment.